Hey, RuinedOne, how's it going?
For what it's worth, I use 3x5 cards for tracking initiative and I wouldn't do it any other way. I checked out the "GameMechanics" cards above and they look pretty spiffy. But I will say that I think it works just as well if you simply use regular old cards. They have the advantage that you don't have to use a printer to run them off beforehand and you can just throw them away.
What I usually do is have "permanent cards" for the PC's and any major NPC's that might be travelling with them. These get updated when the character in question levels. For the bad guys, I just have their stats written in an abbreviated stat block in a notebook beside me and have generic cards that usually say "Bad Guys #1", "Bad Guys #2", etc. When the encounter starts, I'll jot on each Bad Guy card which set of bad guys that card represents (I might have one card for the grunt warrior types, one for the archers and one for the leader for example).
Once the cards get too many scratched out bits of info on them (names of the Bad Guys they represent, damage taken, spell effects, etc.) then I just toss them. Note cards are cheap.